Current-resonant DC power source devices of (SMZ or Soft-switching Multi-resonant Zero-cross) type are widely known as those having their high efficiency and less switching noise. For example, FIG. 20 indicates a prior art current-resonant DC power source device which comprises first and second main MOS-FETs 2 and 3 as a pair of main switching elements connected in series to a DC power source 1; a voltage-resonant capacitor 7 and a series circuit each connected in parallel to first MOS-FET 2, the series circuit having a resonance reactor 4, a primary winding 5a of a transformer 5 and a current-resonant capacitor 6; a first secondary winding 5b of transformer 5; a second secondary winding 5c connected in series to first secondary winding 5a with the same number of turns and same direction of turn; a pair of rectifying diodes 8 and 9 connected in series to opposite ends of first and second secondary windings 5b and 5c with the adverse polarity of first and second rectifying diodes 8 and 9 to each other; a smoothing capacitor 10 connected between a junction of rectifying diodes 8 and 9 and a junction of first and second secondary windings 5b and 5c of transformer 5; a voltage detector 12 for picking up DC output voltage VO applied on an electric load 11 from smoothing capacitor 10; and a control circuit 14 for producing drive signals VG1 and VG2 to main MOS-FETs 2 and 3 for the on-off operation thereof with modulation by detection signals supplied from voltage detector 12 through a photo-coupler 13 to control circuit 14. Parasitic diodes 2a and 3a are connected respectively between drain and source terminals of main MOS-FETs 2 and 3.
In operation of DC power source device shown in FIG. 20, when first and second MOS-FETs 2 and 3 are respectively in the off and on conditions, a resonant current flows from DC power source 1, second MOS-FET 3, resonance reactor 4, primary winding 5a of transformer 5 and current resonant capacitor 6 to DC power source 1 to raise voltage applied on primary winding 5a of transformer 5. When voltage induced on first secondary winding 5b of transformer 5 reaches DC output voltage VO, first rectifying diode 8 is turned on to cause electric current to run from first secondary winding 5b through first rectifying diode 8 to smoothing capacitor 10 which therefore is electrically charged to supply DC power to load 11. At this moment, resonance current ILr by resonance reactor 4 and current-resonant capacitor 6, flows through primary winding 5a of transformer 5. When voltage on primary winding 5a of transformer 5 begins to reduce, voltage developed on first secondary winding 5b begins to descend, and when voltage between both ends of first secondary winding 5b comes to or below DC output voltage VO, first rectifying diode 8 is turned off to stop power supply to secondary side of transformer 5. At the same time, in primary side of transformer 5, resonance current ILr flows through resonance reactor 4, primary winding 5a of transformer 5 and current-resonant capacitor 6 to accumulate electric energy in current-resonant capacitor 6, resonance reactor 4 and primary winding 5a of transformer 5. Then, when second main MOS-FET 3 is turned off under the off condition of first main MOS-FET 2, resonance reactor 4, primary winding 5a of transformer 5 and voltage-resonant capacitor 7 produce a voltage resonance to raise or lower voltage between drain and source terminals of first and second main MOS-FETs 2 and 3 with the voltage inclination determined by the resonance frequency. Thereafter, when first main MOS-FET 2 is turned on while retaining second MOS-FET 3 off, resonance current ILr passes through resonance reactor 4, primary winding 5a of transformer 5 and current-resonant capacitor 6 while discharging energy stored in current-resonant capacitor 6, resonance reactor 4 and primary winding 5a of transformer 5, and resonance current ILr begins to reduce and then flows in the adverse direction. When voltage on primary winding 5a of transformer 5 becomes an anti-polarity and voltage on second secondary winding 5c comes to a level same as that of DC output voltage VO, second rectifying diode 9 is turned on to cause electric current to flow from second secondary winding 5c through second rectifying diode 9 while electrically charging smoothing capacitor 10 and supplying DC power to load 11. As voltage on primary winding 5a of transformer 5 begins to reduce, voltage on second secondary winding 5c concurrently drops. When voltage between both ends of second secondary winding 5c is lowered to or below DC output voltage VO, second rectifying diode 9 is turned off to cease power supply to secondary side of transformer 5 while resonance current flows through resonance reactor 4, primary winding 5a of transformer 5 and current-resonant capacitor 6 in primary side of transformer 5. Continuation of these operations causes repetition of the alternating on-off operation of main MOS-FETs 2 and 3 with the 50% duty ratio. Voltage detector 12 picks out DC output voltage VO applied to load 11 to produce detection signals to control circuit 14 through photo-coupler 13. Control circuit 14 modulates pulse frequency of drive signals VG1 and VG2 supplied to gate terminals of each main MOS-FETs 2 and 3 based on detection signals from voltage detector 12 to control the on-off operation of main MOS-FETs 2 and 3 so as to maintain a substantially constant DC output voltage VO.
In major cases, typical DC power source devices are improved in power conversion efficiency by adopting a synchronous rectification circuit, but in this fashion, the current resonant DC power source shown in FIG. 20 has a trouble in turning a switching element (not shown) in an adopted secondary synchronous rectification circuit on only during the on-period of secondary rectification circuit to pass forward electric current through the rectification circuit since the flowing period of electric current through secondary rectification diodes 8 and 9 is inconsistent with the induced period of voltage on secondary windings 5b and 5c of transformer 5 or the on-period of primary first and second main MOS-FETs 2 and 3. In addition, as an adverse voltage is applied on the switching element in the secondary rectification circuit during the period of time between cease of electric current flowing through the secondary rectification circuit and turning-off of primary first or second main MOS-FET 2 or 3, power conversion efficiency is reduced because an adverse current flows through the switching element in the rectification circuit when the switching element is turned on only during the induced period of voltage on secondary windings 5b and 5c of transformer 5 or during the on-period of primary main MOS-FETs 2 and 3.
To solve the foregoing problem, the following Patent Document 1 discloses a current-resonant switching power source of synchronous rectification type which represses an adverse current flow through a MOS transistor of synchronous rectification type due to back electromotive force of a choke coil connected to a secondary stage before a smoothing capacitor while electric current may flow in the opposite direction through a MOS transistor of synchronous rectification type turned on during the induced period of voltage on the secondary winding of transformer.
[Patent Document 1] Japanese Patent Disclosure No. 11-332233 (on page 4, FIG. 1)